548 
1^. RUGGLES GATES AXD XESTA THOMAS. 
is pulIiDg apart in the same way, while another appears to be 
fragmenting into three pieces, and an additional chromatic 
fragment is found on the spindle. 
Obviously, if pollen grain formation is completed in such 
cases and the pollen grains function, they will produce indi- 
viduals having a different chromatin content in their nuclei. 
This, we believe, would lead to external variations in the off- 
spring. Indeed, all the ineiotic irregularities described in this 
paper may be looked upon as germinal variations, though the 
great majority of them will come to naught because the 
germ-cells, in which they occur, fail to reach maturity on 
account of their aberrant nature. 
The two homotypic spindles in a pollen mother-cell of this 
plant are shown in prophase in fig. 50a and h. They con- 
tain respectively 7 and 8 chromosomes, nearly all of which 
clearly show their bivalent character. They are about to be 
drawn into regular alignment on the equatorial plate. In 
this case the heterotypic segregation was obviously into 7 
and 8 whole chromosomes. In fig. 51 one group of chromo- 
somes in the homotypic metaphase is shown. There are 
present 8 chromosomes and one other which was obviously 
left out of the daughter-nucleus during interkinesis. 
As will be seen from figs. 46-49, there is in this plant 
also great variation in the size of the chromosomes in different 
cells, and irregularities tend to be greater in the less nourished 
cells which have smaller chromosomes. 
During synapsis and diakinesis in QEnothera (Gates ,1908a), 
one, and sometimes two or more nucleoli are present in the 
pollen mother-cell nucleus. When the nuclear membrane 
breaks down in the heterotypic prophase, these nucleoli 
usually disappear immediately, leaving not a trace behind. 
But in certain circumstances they persist for a longer time, 
appearing on or close by the spindle. Fig. 52 shows such a 
case in lata, in which tw^o nucleoli, in addition to the full 
quota of chromosomes, are found on the spindle. These 
nucleoli are distinguishable from the chromosomes by their 
perfectly spherical shape and by their lighter centre. They 
